“If you have healthier soils, they have more soil carbon, but not all soil carbon is equal in terms of its long-term sequestration. Some of this soil is more vulnerable to being lost due to climate or even management changes,” said Caitlin Hicks Pries. “So, how can we know that the carbon that we’re storing is going to withstand the test of time?”
Hicks Pries is an associate professor of biological sciences at Dartmouth College. She discussed the connection between soil health and carbon persistence as part of the Soil Health Indicator Series hosted by the University of Vermont’s Soil Health & Research Extension Center (SHREC).
What’s the difference between particulate carbon and mineral associated carbon?
Organic carbon enters the soil in different ways. One source is plant residues on the soil’s surface such as leaf litter, corn stover, left behind hay and decaying cover crops. Dying roots also contribute to this carbon pool, as do inputs like manure and compost.
These sources of carbon, called particulate carbon, tend to be fast cycling because particulate carbon is easily consumed by soil microbes.
“This carbon is free in the soil. It is unprotected. That means it’s easily accessible by soil microbes,” Hicks Pries said. As the soil microbes consume this particulate carbon, they release carbon dioxide into the atmosphere as a byproduct of their respiration.
There is another pool of carbon, however, that is resistant to this fast cycling process. It’s called mineral associated soil carbon (MASC). The bodies of dead soil microbes contribute significantly to this carbon pool.
Below the soil’s surface, root exudates – organic carbon compounds such as simple sugars, organic acids and amino acids released from living plant roots into the soil – contribute MASC. As water leaches the leaf litter it creates a “tea” which also adds to this more stable carbon pool.
Why is MASC so important?
MASC is carbon in which the molecules are bound to soil minerals. This means that microbes cannot access it. Because of this, MASC tends to be stored for a longer amount of time in the soil and is less vulnerable to change.
In 2021, soil samples (collected to a depth of six inches) were collected from 190 Vermont farms as part of the Vermont State of Soil Health project. Soil samples came from a diversity of production systems: hay, pasture, vegetables, corn and a few wheat fields. Hicks Pries and Erin Lang, a doctoral candidate at Dartmouth, and their lab team tested the samples for minerally associated carbon.
Their goals were two-fold: to see how much soil on Vermont farms is actually protected by minerals and to study how quantities of MASC differ within the various production systems.
Overall, their study found that 60% – 70% of the soil carbon in Vermont is associated with soil minerals. “This is good news. The majority of our carbon is in these forms that can be stored longer term and are more resilient to climate change,” Hicks Pries said.
They did find large differences in the amount of MASC depending on the production systems. Perennial crops (such as hay and pasture) had the greatest concentrations of MASC.
An image from one of the farms where the soil samples were collected. Photo courtesy of Caitlin Hicks Pries
What else did the study reveal about MASC?
As part of the soil health project, all the soil samples underwent the Cornell Comprehensive Assessment of Soil Health (CASH) soil test. The CASH test goes beyond a typical soil test, measuring soil health indicators such as available water capacity, aggregate stability and soil respiration. Hicks Pries and her team looked for correlations between the CASH test results and their results.
One indicator of soil health that went hand in hand with MASC was aggregate stability. An aggregate is a cluster of soil particles, and the more stable the aggregate is, the more it can protect organic carbon from microbes. Aggregate stability is a direct measure of the soil’s capacity to withstand erosion from simulated rainfall. Hicks Pries’s team found that samples with higher aggregate stability had higher levels of MASC.
The other soil health metric that was correlated to MASC was the amount of active carbon – the carbon energy sources available to microbes. Higher concentrations of organic carbon were correlated with significantly more milligrams of MASC.
“It seems that the more active the microbial community, the more of this carbon can be funneled through the microbes and become associated with minerals,” Hicks Pries said.
The team was surprised to find that there appeared to be no relationship between soil pH and the quantity of MASC as long as the pH was above 5.0. “We thought that higher pH soils would have higher microbial activity and would have more mineral associated organic carbon,” Hicks Pries said.
They also found that there is a positive relationship between MASC and soils high in clay and silt. Clay and silt particles provide a large surface area where organic carbon molecules can adhere and become stabilized. As soil temperatures increase in clay and silt soils, it favors microbial activity and favors the funneling of more carbon into mineral associations.
“All of these metrics are really going hand in hand, which is great news. It means you don’t necessarily have to be measuring mineral associated carbon on your farms, but if you have good soil health, you’re more likely to have more of this mineral associated carbon. And that carbon stored in mineral associations will stay in the soil for longer as the climate changes,” Hicks Pries said.
by Sonja Heyck-Merlin
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